The present invention relates generally to equipment utilized in conjunction with a subterranean well and, in an embodiment described herein, more particularly provides an annulus pressure operated electric power generator.
Only a few practical options presently exist for long term provision of electricity to power consuming electric circuits downhole. Batteries and an electric umbilical line extending from the surface to the downhole electric circuit are the most widely implemented of these options. Each of these suffers from some limitations.
An electric umbilical line is exposed to damage during installation and is relatively expensive to install. Batteries which can withstand downhole temperatures are relatively expensive but, unfortunately, are short-lived. Thus, batteries must be replaced periodically.
This periodic replacement requires the downhole assembly to be pulled, or requires the spent batteries to be retrieved separately from the downhole assembly and then replaced with fresh batteries. The former procedure is time-consuming and expensive, and the latter procedure requires an intervention into the well with wireline or slickline equipment.
Thus, it may be seen that it would be very desirable to provide a method of generating electric power downhole to power downhole electric circuits. The electric power generating system would preferably operate using annulus pressure, which is easily controllable from the surface.
In carrying out the principles of the present invention, in accordance with an embodiment thereof, an annulus pressure operated electric power generator is provided. An electric generating system uses increases and decreases in annulus pressure to generate electric power. Methods of generating electric power downhole are also provided.
In one aspect of the invention, an electric power generating system is provided in which fluid flow into and out of an accumulator in response to pressure increase and pressure decrease, respectively, in an annulus is used to drive a generator. For example, the generator may generate direct current having one polarity when fluid flows into the accumulator, and the generator may generate direct current having an opposite polarity when fluid flows out of the accumulator. The generator may be driven by a turbine, by a mechanical linkage, or by other means. Alternatively, the generator may include separate portions, such as a coil and magnets, which are displaced relative to one another to generate electricity.
In another aspect of the invention, an electric power generating system is provided in which pressure increases and decreases in an annulus displace a piston. Displacement of the piston forces fluid to circulate through a hydraulic circuit. A turbine is interconnected in the hydraulic circuit so that, when fluid flows through the circuit, the turbine rotates. Turbine rotation drives a generator, which produces electricity.
In yet another aspect of the invention, a method is provided in which electric power is generated when annulus pressure is increased, and electric power is generated when annulus pressure is decreased. The electric power may be generated in direct current form, and the polarity (i.e., direction of current flow) may be opposite between annulus pressure increases and annulus pressure decreases. In that case, a full wave rectifier may be used to produce a consistent current flow direction for a downhole electric circuit. The electric power may alternatively be generated in alternating current form, whether annulus pressure is increased or decreased.
These and other features, advantages, benefits and objects of the present invention will become apparent to one of ordinary skill in the art upon careful consideration of the detailed description of representative embodiments of the invention hereinbelow and the accompanying drawings.